1. Field of the Invention
Embodiments of the present invention relate generally to a method and apparatus for retaining polishing material in a polishing system.
2. Background of the Invention
In semiconductor wafer processing, the use of chemical mechanical planarization, or CMP, has gained favor due to the enhanced ability to increase device density on a semiconductor workpiece, or substrate, such as a wafer. As the demand for planarization of layers formed on wafers in semiconductor fabrication increases, the requirement for greater system (i.e., process tool) throughput with less wafer damage and enhanced wafer planarization has also increased.
Chemical mechanical planarization systems generally retain a substrate, or semiconductor wafer, in a polishing head. The polishing head presses the substrate against a polishing material. The polishing head and polishing material are moved relative to each other providing a polishing motion between the substrate and the polishing material. A polishing fluid is disposed between the substrate and the polishing material during polishing. The polishing fluid, which may contain abrasives, generally provides the chemical activity at the substrate""s surface that aids in the removal of material from the substrate.
One type of polishing material that may be utilized for chemical mechanical polishing is known as a fixed abrasive material. The fixed abrasive material comprises a plurality of abrasive particles suspended in a resin binder that is disposed in discrete elements on a backing sheet. As the abrasive particles are contained in the polishing material itself, systems utilizing fixed abrasive material generally use polishing fluids that do not contain abrasives.
Fixed abrasive polishing material is generally available in stick-down form but is often utilized in the form of a web. Generally, the web is periodically advanced over the course of polishing a number of substrates as the polishing surface of the web is consumed by the polishing process. A vacuum is typically applied between the web and an underlying support to fix the web to the support during the polishing process. Before the web is advanced, the vacuum is removed, freeing the web from the support surface.
A problem that is periodically encountered when using webs of polishing material is maintaining the web flush with the surface of the polishing platen. For example, the web may creep in relation to the plate during polishing due to the shear forces experienced as the web and platen move in relation to a substrate that is held against the web during polishing. As the web creeps, it may wrinkle, creating a non-planar surface that may disrupt the polishing uniformity. Moreover, wrinkles extending across the vacuum sealing area at the edge of the platen and the web may cause seal failure, thus allowing the web to separate from the platen.
Additionally, bubbles of air or other gases trapped between the web and platen reduce the surface contact area between the web and platen. The reduced contact area correspondingly decreases frictional forces between the web and platen, thus disadvantageously increasing the probability that the web may creep across the platen. Moreover, wrinkles in the web tend to aggravate bubble formation between the web and platen. Bubbles under the web change the modulus of the integrated polishing material (i.e., the polishing material in combination with supporting materials) surface that directly alters the polishing results, thus creating a condition that undesirably leads to unpredictable polishing results and premature consumption of the web of polishing material. Stick-down pads experience similar problems although to a lesser extent due to increased frequency of releasing and re-securing the polishing material in web based systems.
Therefore, there is a need for a platen that improves the retention of polishing material thereon.
One aspect of the present invention generally provides an apparatus for retaining polishing material. In one embodiment, the apparatus includes a platen having a top surface, a plurality of channels and one or more vacuum ports. The top surface is adapted to support the polishing material. The plurality of channels are formed in a polishing area of the top surface. The vacuum ports are disposed in the platen and at least one port is in communication with at least one of the channels.
In another aspect of the invention, a method for retaining polishing material to a platen having at op surface that includes a network of channels bounded by a groove is provided. In one embodiment, the method includes the steps of applying vacuum to the groove disposed in the top surface, drawing a portion of fluid trapped between the polishing material and platen inward of the groove through a plurality of channels to the groove, and drawing another portion of fluid trapped between the polishing material and the platen through a plurality of vacuum ports coupled to the channels.